This invention relates to an AC motor control system and, more particularly, to an AC motor control system in which a region of constant output power can be enlarged in cases where the AC motor output power is limited in magnitude.
AC motors (induction motors) develop a constant output (constant output characteristic) at speeds above a certain constant speed (base speed), and develop a constant torque (constant torque characteristic) at speeds below the base speed. This may be understood from FIG. 1, which shows the characteristics of an AC motor. The solid line indicates the output power (KVA)-speed characteristic, and the broken line the torque-speed characteristic. Nb indicates the base speed. In AC motors of this kind, the following three advantages would manifest themselves if it were possible to decrease the AC motor output power with a decrease in load and widen the region of the constant output characteristic:
(1) the power supply output (KVA) can be reduced in proportion to the output power of the AC motor, making it possible to reduce the power supply capacity for small loads even if the capacity of the AC motor is large;
(2) the number of gear stages can be reduced in cases where the AC motor is employed as a spindle motor in a small-size machining center; and
(3) in the constant peripheral speed control of a lathe or the like, cutting can be performed while exploiting the output power of the motor to the maximum extent by widening the region of the constant output characteristic.
Hereinafter, control executed for the purpose of reducing AC motor output power in accordance with the size of a load and widening the constant output characteristic region shall be referred to as output limitation control.
If we assume that a voltage-speed characteristic is as represented by the solid line in FIG. 2 without application of output limitation control, then the desired characteristic would be as shown by the dot-and-dash line if output limitation control were applied. With methods according to the prior art, however, the output-speed characteristic actually obtained is as depicted by the dashed line, even with output limitation control. It will be appreciated that the region of constant output power cannot be widened significantly according to the prior art, making it impossible to realize the three advantages set forth above.